1. Field of the Invention
This invention relates to a method for the production of a particulate hydrogel polymer and an absorbent resin. More particularly, it relates to a method for the production of a particulate hydrogel polymer, characterized by continuing application of a load of a magnitude in a specific range on a hydrogel polymer possessing a cross-linked structure at a specific temperature and meanwhile repeating exertion of a shear force thereon. It further relates to a method for the production of an absorbent resin by further drying the particulate hydrogel polymer.
2. Description of the Prior Art
As absorbent resins, cross-linked polyacrylates, saponified acrylic ester-vinyl acetate copolymers, modified cross-linked polyvinyl alcohol, cross-linked isobutylenemaleic anhydride copolymer, starch-acrylic acid graft polymer, etc. have been known to the art. These absorbent resins have been finding extensive utility in applications to such hygienic absorbents as hygienic napkins and disposable diapers and to water-retaining agents and dehydrating agents in the agricultural-horticultural field and the civil engineering field.
As respects methods for the production of absorbent resins, those resorting to reversed-phase suspension polymerization have been disclosed in JP-A-57-161,408(1981), JP-A-59-94,011(1982), JP-A-57-158,209(1982), and JP-A-57-198,714(1982), for example, and those resorting to aqueous solution polymerization have been disclosed in JP-A-59-34,101(1982), JP-B-48-42,466(1973), JP-A-58-49,714(1983), and JP-B-59-37,003(1984), U.S. Pat. No. 4,286,082 and U.S. Pat. No. 4,625,001, for example.
The methods of reversed-phase suspension polymerization, however, require use of an organic solvent, consequently not only impair work environment but also threaten the possibility of fire and explosion, necessitate adoption of a countermeasure, and prove highly expensive on account of the cost for the organic solvent coupled with the cost for removal of the used organic solvent. Moreover, since this organic solvent persists, though in a minute amount, in the produced polymer, the inevitable work for thorough removal of the organic solvent further boosts the cost of production of the polymer. The absorbent resin produced by the reversed-phase suspension polymerization comprises spherical particles of small diameters. When this absorbent resin is used in a disposable diaper, for example, the handling of this diaper is inconvenient because the minute spherical particles of the resin deposited on fibrous absorption cores such as of pulp are not retained fast thereto but are readily separated therefrom.
In contrast, the methods of aqueous solution polymerization disclosed in JP-A-57-34,101(1982) and U.S. Pat. No. 4,625,001 are free from the problem described above. These methods produce a cross-linked polymer by subjecting the aqueous solution of a monomer destined to form a cross-linked structure and grow into a hydrogel polymer in the process of aqueous solution polymerization and a polymerization initiator to radical solution polymerization in a reaction vessel provided with stirring vanes and continuing the radical solution polymerization while finely dividing the hydrogel polymer being formed in consequence of the polymerization by dint of the shear force generated by the rotation of the stirring vanes around the axis thereof. These methods are advantageous in that their operational efficiency is exceptionally high and a finely divided hydrogel polymer having a cross-linked structure in the molecular unit thereof is produced in a high efficiency. Even these methods have the possibility of betraying inferiority of operational efficiency in the production of an absorbent resin having a high absorption ratio and containing a water-soluble substance in a low concentration.
The phenomenon that an absorbent resin increases an absorption ratio thereof in proportion as the cross-link density thereof decreases is well known to persons of ordinary skill in the art. It is also known that an absorbent resin produced by a procedure involving the work of lowering the cross-link density inevitably contains a water-soluble substance in an unduly large concentration. The water-soluble substance is of such nature that when the absorbent resin forms a hydrogel structure on contact with a liquid such as water, urine, or body fluid which is subjected to absorption, this substance is exudated from the hydrogel structure. The water-soluble substance which is extracted by the liquid being absorbed not merely lowers the absorption ratio of the absorbent resin but also aggravates the deterioration of the absorbent resin. Moreover, the ropy substance which the water-soluble substance produces on contact with a liquid induces an objectionable situation as by conferring an unpleasant feeling on the user's skin and defiling the liquid subjected to absorption, for example.
A desire, therefore, has been expressed for a method which is capable of producing an absorbent resin having a high absorption ratio and containing a water-soluble substance only in a low concentration.
U.S. Pat. No. 4,654,039 and JP-A-1-144,404(1990) propose methods for producing an absorbent resin having a high absorption ratio and containing a water-soluble substance in a low concentration by subjecting to aqueous solution polymerization a monomer having a free acid type structure or exhibiting a specific neutralization ratio. These methods of production, however, are disadvantageous in that they require an aftertreatment for neutralization, they entail a complicated operation and suffer from poor productivity, and they impose restrictions on conditions of polymerization.
The hydrogel polymer which is obtained by polymerization is generally subjected to a drying treatment and then pulverized and marketed as a powdered product. Heretofore, for the purpose of efficiently carrying out this drying of the hydrogel polymer, devices have been employed for increasing the surface area of the hydrogel polymer to the fullest possible extent. A method for pulverizing a hydrogel polymer by extruding this polymer through a perforated plate (JP-B-54-32,176(1979), JP-A-50-136,348(1975), etc.) has been known, for example. By this known method, hydrogel polymer particles ideal for the utility aimed at cannot be obtained because the extrusion disintegrates the hydrogel polymer into a fine powder which possibly reconglomerates and gives rise to a ropy substance.
A method which uses such an additive as a lubricant for the purpose of precluding the possible reconglomeration of disintegrated hydrogel polymer in the operation of pulverizing a hydrogel polymer by extrusion through a perforated plate (JP-A-59-30,826(1984) and JP-59-119,172(1984)) has been known. This method has the possibility that the additive suffered to remain in the produced polymer will manifest an adverse effect on the performance of a finished product of the polymer.
The aforementioned methods disclosed in JP-A-57-34,101(1982) and U.S. Pat. No. 4,625,001 are capable of producing hydrogel polymer particles of relatively small diameters. These hydrogel polymer particles have the disadvantage that they contain coarse particles measuring not less than 10 mm in size in a proportion of several % by weight to some tens of % by weight, have a wide particle size distribution, and betray deficiency in uniformity of drying efficiency and degree of drying.
Further, by the conventional method for pulverization of the hydrogel polymer, the production of hydrogel polymer particles of a sufficiently small average diameter has been obtained only with difficulty in a conspicuously low efficiency. When hodrogel polymer particles are dried to afford a finished product of particles of a desired size, the dried hydrogel polymer particles must be pulverized if the original hydrogel polymer particles have a large average particle diameter. The pulverization gives rise to minute particles which are smaller than the desired size. It is universally known to persons of ordinary skill in the art that these minute particles are undesirable in point of performance of the absorbent resin and convenience of handling.
No method has yet been established for enabling hydrogel polymer particles and an absorbent resin which have a high absorption ratio and contain a water-soluble substance only in a small concentration to be produced in a high operational efficiency by a convenient process. Neither has there been established a method capable of highly efficiently producing hydrogel polymer particles which contain no such additive as a lubricant, exhibit a narrow particle size distribution, and excel in efficiency of drying. A method capable of efficiently producing hydrogel polymer particles having a sufficiently small average diameter remains yet to be established.
An object of this invention, therefore, is to provide a method for producing hydrogel polymer particles and an absorbent resin having a high absorption ratio and containing a water-soluble substance only in a small concentration.
Another object of this invention is to provide a method for enabling hydrogel polymer particles and an absorbent resin having a high absorption ratio and containing a water-soluble substance only in a small concentration to be produced in a high operational efficiency by a convenient process.
Still another object of this invention is to provide a method for highly efficiently producing hydrogel polymer particles containing no such additive as a lubricant, exhibiting a narrow particle size distribution, and excelling in efficiency of drying.
Yet another object of this invention is to provide a method for producing highly efficiently hydrogel polymer particles having a sufficiently small average particle diameter.